The Failure Mechanism of 3D C/C Composite Under Compression-Shear Coupled Loads: An Experimental Study

Material strength under complex stress states is vital for structure design. This paper studied the strength and failure behaviour of 3D C/C composites under compression-shear coupled loads. Experiments were conducted using a modified anti-symmetric four-point bending (MAFPB) method and off-axis compression method. Two dominant failure modes were observed; 1) shear and 2) compression. Mode transitions under relatively low shear/compressive rations were also observed. Results show that the material exhibits significantly lower failure stress under shear-compressive load compared to compressive or shear strengths alone. Further analysis revealed that meso-scale geometry characteristic including tow crimps and interfacial cracks are the main inducements: (1) tow crimps lead to local bending moment and increase local shear stress, (2) matrix-tow cracks formed under shear load degrade the lateral support of axial tows, (3) matrix fibre splitting and local buckling within tows induced by compressive load further reduce the shear strength of axial tows. Failure stresses in off-axis tests are lower compared to those in MAFPB tests due to the existence of bi-axial compression. These findings show that shear failure is a weakness for the 3D C/C composite and can provide insights for further material design and structural strength analysis.